1. Field of the Invention
The invention relates generally to digital amplifiers and more particularly to systems and methods for implementing over-current protection in all-digital amplifiers while using simple, low-cost current sensing mechanisms.
2. Related Art
Audio amplifiers are designed to receive input signals that typically have very low voltages and/or currents, and to generate corresponding output signals that generally have much higher voltages and/or currents. Although these higher voltages/currents are necessary to drive speakers and thereby generate an audible signal, this also presents a danger to the speakers. In other words, if the voltage/current is too high, the speakers could be damaged.
In a pulse-width modulation (PWM) amplifier, preventing the amplifier from generating excessive output current (creating an over-current condition) is one of the most critical functions, since this could damage the amplifier's output stages or the speakers driven by the output stages. The amplifier can avoid over-current conditions is various ways. Probably the most straightforward solution is to simply shut down the system whenever an over-current condition occurs. While this approach is effective to prevent the current from reaching damaging levels, as a practical matter it may not be an acceptable solution. For instance, some audio contents will cause short term over-current conditions that are not damaging to the output stages or speakers. In this scenario, shutting down the system does not avoid a damaging condition, but does interrupt the audio output of the system. Obviously, this is not desirable.
In another situation, the input audio signal will result in an over-current condition that is more persistent than the short-term condition described above. This condition may require that some corrective action be taken, but is not so serious that the system must be immediately shut down to avoid damage to the speakers or the system itself. In this situation, it would be preferable for the PWM amplifier to continue to provide audio output and to transition gracefully between a normal operating mode and a corrective operating mode. That is, the transition (as well as the corrective operating mode) should not be accompanied by side effects that significantly impact the performance of the system or produce artifacts in the audible output of the system. If the corrective operating mode is not sufficient to eliminate the over-current condition, the system may be shut down to avoid damage.
It is difficult in a digital amplifier to design an over-current protection system that is both a low-cost solution and meets the above goals. The need for low cost favors the straightforward solution of shutting down the system whenever the output current exceeds a predetermined threshold and resuming operation when the current falls below the threshold. As noted above, this may cause unnecessary interruption of the output audio signal. Moreover, if the system reacts too quickly to the current exceeding and then falling below the threshold, the actual switch rate of the FETs may increase, which may in turn lead to increased heating, which may then damage the FETs. If, on the other hand, the protection system reacts too slowly, the current may reach damaging levels before shut-down occurs. Alternatively, there may be oscillation. That is, when shut-down occurs, the current may drop too much before operation is resumed, then current may ramp back up so that it is too high before shut-down again occurs. Depending on the actual speed of the protection system, the oscillation may be in the audio range, or it may cause other audible effects.
It would therefore be desirable to provide systems and methods for protecting a digital amplifier from over-current conditions, where a simple, low-cost current sensing mechanism is used, but the response to over-current conditions can be more sophisticated than simply shutting down the amplifier.